1. Field of the Invention
The present invention relates to a surface acoustic wave filter device, for use as, for example, a band-pass filter provided in, for example, portable telephones. More particularly, the present invention relates to a surface acoustic wave filter device in which the characteristic impedance on the input side differs from the characteristic impedance on the output side and which has an unbalanced-to-balanced conversion function.
2. Description of the Related Art
In recent years, to decrease the size of portable telephones, composite components in which the number of component parts and the sizes thereof are reduced and which have multiple functions have been developed.
In such a surface acoustic wave filter used in the RF stage of a portable telephone, one having an unbalanced-to-balanced conversion function has been used.
In a portable telephone, the portion from the antenna to the surface acoustic wave filter as a band-pass filter is defined by an unbalanced circuit, and generally has a characteristic impedance of 50 xcexa9. In contrast, an amplifier used in a later stage of the surface acoustic wave filter has a balanced terminal and often has an impedance of 150 xcexa9 to 200 xcexa9. Therefore, a surface acoustic wave filter device having a balanced-to-unbalanced conversion function that converts an unbalanced input of 50 xcexa9 into a balanced output of 150 xcexa9 to 200 xcexa9 has been used.
An example of this type of surface acoustic wave filter is disclosed in Japanese Unexamined Patent Application Publication No. 10-117123. Referring to FIG. 19, this conventional surface acoustic wave filter device is described below. In a surface acoustic wave filter device 500, surface acoustic wave filters 511 and 512 are connected to an unbalanced input terminal 500a. The surface acoustic wave filter 511 is configured such that the surface acoustic wave filters 511a and 511b are cascaded in two steps. Similarly, the surface acoustic wave filter 512 is configured such that the surface acoustic wave filters 512a and 512b are cascaded in two steps. The output ends of the surface acoustic wave filters 511 and 512 are provided as balanced output terminals 500b and 500c, respectively.
In the filter having a balanced-to-unbalanced conversion function, there is a demand for the transmission characteristics in the pass band between an unbalanced terminal and one of the balanced terminals to have an equal amplitude and a phase which is inverted by 180xc2x0 with respect to the transmission characteristics in the pass band between the unbalanced terminal and the other balanced terminal. Such relationships of amplitude and phase between two transmission characteristics are called xe2x80x9cdegree of amplitude balancexe2x80x9d and xe2x80x9cdegree of phase balancexe2x80x9d, respectively.
The above-described degree of amplitude balance and degree of phase balance are expressed by the following equations when a filter device having balanced-to-unbalanced input/outputs is a device of three ports and when the unbalanced input terminal is denoted as xe2x80x9cport 1xe2x80x9d and the balanced output terminals are denoted as xe2x80x9cport 2xe2x80x9d and xe2x80x9cport 3xe2x80x9d:
Degree of amplitude balance=|A|
where A=|20xc3x97log S21|xe2x88x92|20xc3x97log S31|, and where S21 represents the amplitude of the transmission characteristics between port 1 and port 2, and S31 represents the amplitude of the transmission characteristics between port 1 and port 3.
Degree of phase balance=|Bxe2x88x92180|
where B=|∠S21xe2x88x92∠S31|, and where ∠S21 represents the phase between port 1 and port 2, and ∠S31 represents the phase between port 1 and port 3.
Ideally, in the pass band of the filter, the degree of amplitude balance is 0 dB and the degree of phase balance is zero degrees. However, in practice, the degree of amplitude balance only needs to be 1.5 dB or less, and the degree of phase balance only needs to be 20 degrees or less. Preferably, the degree of amplitude balance is 1.0 dB or less, and the degree of phase balance is 10 degrees or less.
In the conventional surface acoustic wave filter having a balanced-to-unbalanced conversion function, such as the above-described Japanese Unexamined Patent Application Publication No. 10-117123, to invert the phase of the transmission characteristics between an unbalanced terminal and one of the balanced terminals with respect to the transmission characteristics between the unbalanced terminal and the other balanced terminal, a method in which the polarity of an IDT (Interdigital Transducer) is inverted or a method in which, when the wavelength of the surface wave is denoted as xcex, the IDT-to-IDT spacing is widened by 0.5xcex, is used. As a consequence, a difference occurs in the electrical characteristics due to the difference between the configuration between an unbalanced terminal and one of the balanced terminals and the configuration between the unbalanced terminal and the other balanced terminal. Therefore, the degree of amplitude balance and the degree of phase balance in the pass band are often unsatisfactory.
Furthermore, in the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 10-117123, to prevent the deterioration of the above-described degree of balance, four surface acoustic wave filter devices 511a, 511b, 512a, and 512b are used, and each of the surface acoustic wave filter devices 511 and 512 has a two-step configuration. As a result, the size of the surface acoustic wave filter device 500 is substantially increased, and thus, size reduction is difficult. Furthermore, since the number of surface acoustic wave filter devices which can be manufactured from one wafer is decreased, the manufacturing costs substantially increase.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a surface acoustic wave filter device which has a balanced-to-unbalanced conversion function, in which input and output impedances are different, which has a satisfactory degree of amplitude balance and degree of phase balance, and having a greatly reduced size and cost.
According to a first preferred embodiment of the present invention, a surface acoustic wave filter device includes a first surface acoustic wave filter element having a plurality of IDTs provided along the propagation direction of a surface-acoustic wave on a piezoelectric substrate and having first and second terminals, and a second surface acoustic wave filter element having a plurality of IDTs arranged along the propagation direction of a surface-acoustic wave on a piezoelectric substrate, the spacing between input/output IDTs is greater than that of the first surface acoustic wave filter element by approximately half the wavelength of the surface-acoustic wave, and having first and second terminals, the first terminals of the first and second surface acoustic wave filter elements being commonly connected to an unbalanced terminal, and the second terminals of the first and second surface acoustic wave filter elements being connected to a balanced terminal, wherein the total of the electrostatic capacitance values of the plurality of IDTs of the second surface acoustic wave filter element is greater than the total of the electrostatic capacitance values of the plurality of IDTs of the first surface acoustic wave filter element.
In the surface acoustic wave filter device in accordance with the first preferred embodiment of the present invention, when the total of the electrostatic capacitance values of the plurality of IDTs of the second surface acoustic wave filter element is denoted as C2 and when the total of the electrostatic capacitance values of the plurality of IDTs of the first surface acoustic wave filter element is denoted as C1, the condition C1 less than C2 less than 1.20(C1) is preferably satisfied.
When the amount of electrode coverage in the plurality of IDTs of the first surface acoustic wave filter element is denoted as M1, the electrode finger crossing width is denoted as W1, the total number of pairs of the electrode fingers of the plurality of IDTs is denoted as N1, the amount of electrode coverage in the plurality of IDTs of the second surface acoustic wave filter element is denoted as M2, the electrode finger crossing width is denoted as W2, the total number of pairs of the electrode fingers of the plurality of IDTs is denoted as N2, the condition M1xc3x97W1xc3x97N1 less than M2xc3x97W2xc3x97N2 less than 1.20xc3x97M1xc3x97W1xc3x97N1 is satisfied, and more preferably, the condition 0.93xc3x97M1 less than M2 less than 1.05xc3x97M1 is preferably satisfied.
The first and second surface acoustic wave filter elements have reflectors on both sides of the area where the IDTs are located, when the spacing between the reflector of the first surface acoustic wave filter element and the IDT adjacent to the reflector is denoted as GR1, and when the spacing between the reflector of the second surface acoustic wave filter element and the IDT adjacent to the reflector is denoted as GR2, the condition 0.96GR1 less than GR2  less than 1.02GR1 is preferably satisfied.
According to a second preferred embodiment of the present invention, a surface acoustic wave filter device includes a first surface acoustic wave filter element having a plurality of IDTs arranged along the propagation direction of a surface-acoustic wave on a piezoelectric substrate and having first and second terminals, and a second surface acoustic wave filter element having a plurality of IDTs arranged along the propagation direction of a surface-acoustic wave on a piezoelectric substrate, the spacing between input/output IDTs is greater than that of the first surface acoustic wave filter element by approximately half the wavelength of the surface-acoustic wave, and having first and second terminals, the first terminals of the first and second surface acoustic wave filter elements being commonly connected to an unbalanced terminal, and the second terminals of the first and second surface acoustic wave filter elements being connected to a balanced terminal, wherein, in at least one of the first and second surface acoustic wave filter elements, at least one IDT-to-IDT spacing is different from the other IDT-to-IDT spacings by an integer multiple of the wavelength of the surface-acoustic wave.
In the first surface acoustic wave filter element, at least one IDT-to-IDT spacing is different from the other IDT-to-IDT spacings by an integer multiple of the wavelength of the surface-acoustic wave, and in the second surface acoustic wave filter element, a plurality of the IDT-to-IDT spacings are greater than the smallest IDT-to-IDT spacing of the first surface acoustic wave filter element and are less than the largest IDT-to-IDT spacing of the first surface acoustic wave filter element.
In the first surface acoustic wave filter element, at least one IDT-to-IDT spacing is different from the other IDT-to-IDT spacings by approximately one wavelength of the surface-acoustic wave, and in the second surface acoustic wave filter element, the plurality of IDT-to-IDT spacings have substantially the same value and are greater by approximately half the wavelength of the surface-acoustic wave than the smallest IDT-to-IDT spacing of the first surface acoustic wave filter element.
In the surface acoustic wave filter device in accordance with the first and second preferred embodiments of the present invention, the surface acoustic wave filter device further includes surface acoustic wave resonators which are connected in parallel or in series on at least one of the balanced terminal side and the unbalanced terminal side.
In the surface acoustic wave filter device in accordance with preferred embodiments of the present invention, the surface acoustic wave filter device also preferably includes surface acoustic wave resonator-type filters which are cascaded on at least one of the balanced terminal side and the unbalanced terminal side.
A communication device in accordance with another preferred embodiment of the present invention includes a surface acoustic wave filter device according any of the preferred embodiments of the present invention described above.
In the surface acoustic wave filter device according to the first preferred embodiment of the present invention, in a configuration in which the second surface acoustic wave filter element is arranged such that the spacing between input/output IDTs is greater than that of the first surface acoustic wave filter element by approximately half the wavelength of the surface-acoustic wave, and the phase characteristics are inverted, the difference in the impedance characteristics, which occurs as a result differing the IDT-to-IDT spacing, is corrected by making the total of the electrostatic-capacitance values of the second surface acoustic wave filter elements greater than the total of the electrostatic-capacitance values of the first surface acoustic wave filter elements.
Therefore, a surface acoustic wave filter device having a balanced-to-unbalanced conversion function, in which the in-band degree of amplitude balance is satisfactory, is provided. As a result, a surface acoustic wave filter device is provided which has a balanced-to-unbalanced conversion function, in which the degree of balance is satisfactory and in which input and output impedances are different.
In particular, when the total of the electrostatic capacitance values of the plurality of IDTs of the second surface acoustic wave filter element is denoted as C2 and when the total of the electrostatic capacitance values of the plurality of IDTs of the first surface acoustic wave filter element is denoted as C1, the degree of amplitude balance is effectively improved when the condition C1 less than C2 less than 1.20(C1) is satisfied. In a similar manner, when the amount of electrode coverage in the plurality of IDTs of the first surface acoustic wave filter element is denoted as M1, the electrode finger crossing width is denoted as W1, the total number of pairs of electrode fingers is denoted as N1, the amount of electrode coverage in the plurality of IDTs of the second surface acoustic wave filter element is denoted as M2, the electrode finger crossing width is denoted as W2, the total number of pairs of electrode fingers of the plurality of IDTs is denoted as N2, the degree-of-balance is effectively improved similarly when the condition M1xc3x97W1xc3x97N1 less than M2xc3x97W2xc3x97N2 less than 1.20xc3x97M1xc3x97W1xc3x97N1 is satisfied, and the degree-of-balance is further improved when the condition 0.93xc3x97M1 less than M2 less than 1.05xc3x97M1 is satisfied.
Furthermore, the degree of phase balance is also effectively improved when the spacings GR1 and GR2 between the reflectors in the first and second surface acoustic wave filter elements and the IDT adjacent to the reflector are set such that 0.96GR1 less than GR2 less than 1.02GR1.
In the surface acoustic wave filter device in accordance with the second preferred embodiment of the present invention, since the IDT-to-IDT spacing of the first surface acoustic wave filter element differs by one wavelength between the right and left, and the IDT-to-IDT spacing of the second surface acoustic wave filter element is greater by approximately half the wavelengths than the smallest one of the IDT-to-IDT spacing of the first surface acoustic wave filter elements between the right and left, the difference in the impedance characteristics due to the IDT-to-IDT spacing is averaged. Therefore, the impedance characteristics of the first and second surface acoustic wave filter elements substantially match each other, and the degree of amplitude balance is effectively maintained. As a result, according to the second preferred embodiment of the present invention, a surface acoustic wave filter device has a balanced-to-unbalanced conversion function, in which the degree of balance is satisfactory and in which input and output impedances are different.
In the present invention, when surface acoustic wave resonators are connected in parallel or in series on at least one of the balanced terminal side and the unbalanced terminal side, the amount of out-of-band attenuation is greatly increased without deteriorating the in-band insertion loss. Similarly, in a case where surface acoustic wave resonator-type filters are connected on at least one of the balanced terminal side and the unbalanced terminal side, the amount of out-of-band attenuation is greatly increased.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.